Acquired immune deficiency syndrome (AIDS) is a fatal disease of growing prevalence in the modern world. The agent responsible for this disease, human immunodeficiency virus (HIV), was first identified in 1983. HIV is a T-lymphotropic retrovirus that invades and replicates in cells of the immune system, primarily helper T-lymphocytes. The consequent dysfunction in T-lymphocyte-mediated immunity results in an immuno-compromised condition. Patients usually die of associated opportunistic viral, bacterial or fungal infections. A characteristics laboratory finding in AIDS is the decrease in helper T lymphocytes (CD4), and particularly a steady decrease in the ratio of CD4 to suppressor T lymphocytes CD8 as the disease progresses. Virus binding is primarily mediated by interaction of gp120, the external subunit of the HIV envelope glycoprotein (Env) with CD4 protein and various coreceptor molecules (one of several alternative chemokine receptors). These interactions then activate the gp41 transmembrane subunit of the envelope glycoprotein, to cause fusion between the virus and cell membranes. See Retroviruses, Coffin et al. (eds.) (1997) CSHP, New York, Ch. 11.
The humoral immune system is triggered by HIV infection, though it generally does not provide sufficient protection to ward off the infection. Env is the major target of anti-HIV neutralizing antibodies (Wyatt et al. Nature 393:705-711, 1998). However, Env has evolved so that its relatively invariant neutralizing determinants are protected from the humoral immune system. Antibodies to these regions therefore are generated at a low frequency and their neutralizing activities in vivo are generally weak. Certain variable regions (e.g., the V3 loop) are targets for potent neutralizing antibodies, but these are typically restricted to a limited number of HIV-strains (in other words, they are not broadly cross-reactive). For a list of several gp120 antigenic epitopes and consensus definitions of the conserved and variable regions of gp120, see published PCT application PCT/US98/02766 (publication number WO 98/36087) and Coffin et al. (eds.) (1997) CSHP, New York, Ch. 12.
A neutralizing monoclonal antibody (MAb) with potent and broadly cross-reactive activity would have great potential value in protocols aimed at preventing HIV infection before or immediately after exposure, for example in neonatal transmission, post-exposure prophylaxis, and as a topical inhibitor. Such a MAb may also be useful in treating chronic infection (D'Souza et al. J. Infect. Dis. 175:1056-1062, 1997). However only a handful of MAbs with the desired broadly cross-reactive neutralizing activities have been described. Because of limited potency and cross-reactivity of these molecules, even the three most promising candidates have questionable clinical value (D'Souza et al., 1997).
Extensive efforts are underway to provide immunological or pharmacological approaches to controlling HIV infection (Coffin et al., 1997, Ch. 12). The specific interaction between gp120 and CD4 has been exploited in efforts to provide a possible treatment for HIV infection. See, e.g., U.S. Pat. No. 5,817,767; Capon et al., Nature 337:525-531, 1989. A soluble fragment of CD4 (sCD4), comprising the first and second domains of this protein (D1D2) has been generated, and this molecule interacts specifically with gp120, essentially serving as a molecular decoy. sCD4 has been shown to block the spread of HIV between cultured cells (Moore et al., Science 250:1139-1142, 1990). However, clinical trials with sCD4 were inconclusive as to the effects on human viral load (Schooley et al., Ann. Internal Med. 112:247-253, 1990; Kahn et al., Ann. Internal Med. 112:254-261, 1990). Subsequent studies indicated that, unlike laboratory-adapted HIV strains, isolates obtained directly from infected patients (primary isolates) are resistant to neutralization by sCD4 (Darr et al., Proc. Natl. Acad. Sci. 87:6574-6578, 1990).
In another approach, researchers have generated an antibody-like molecule by fusing the binding portion of CD4 to the constant region (Fc) of a human IgG heavy chain (see, e.g., Capon et al., Nature 337:525-531, 1989; and Byrn et al., Nature 344:667-670, 1990). This molecule, termed CD4 immunoadhesin, exploits the native functions of immunoglobulin Fc, such as its ability to fix complement, its ability to mediate antibody-dependent cytotoxicity, and its transfer across the placental barrier. There are significant drawbacks to using Fc receptors in association with CD4, because such a construct may be responsible for targeting HIV to Fc-receptor bearing cells (e.g. macrophages), and might lead to increased transmission of HIV-1 across the placental barrier.
A complementary recombinant molecule has also been made, wherein the binding portion of CD4 is fused to the Fv region of an antibody directed to human CD3; this “Janusin” molecule may be able to re-target cytotoxic T-lymphocytes onto HIV-infected cells (Traunecker et al., Embo J. 10:3655-3659, 1991; Traunecker et al., Int. J. Cancer: Supp. 7:51-52, 1992). Janusin has been reported to inhibit HIV-mediated cell fusion when administered in vitro with neutralizing antibody to either gp41 or the V3 loop of gp120 (Allawayetal., AIDS Res. Hum. Retroviruses 9:581-587, 1993; U.S. Pat. No. 5,817,767). This system is inherently complicated and inefficient because multiple molecules must be co-administered to the subject.
This invention is directed to proteins that address key failures of the prior art.